Include an exercise for qcut()

**Describe Topic**
Include an exercise to implement the `qcut()` function in datatables which is a function for binning columns

**Suggested Set**
It can possibly be included in **Set 5: Column Aggregations**

**Sample Exercise**
Exercise : Create a new column B in frame data which contains deciles of column A
Solution : `data['B'] = dt.qcut([dt.f.A])`